Radiation sensitive display system utilizing a cholesteric liquid crystalline phase material



Sept. 10, 1968 J. L. FERGASON ET AL 3, 6

RADIATION SENSITIVE DISPLAY SYSTEM UTILIZING A CHOLESTERIC LIQUID CRYSTALLINE PHASE MATERIAL Filed June 29, 1965 2 Sheets-$heet 1 7 VIDEO SOURCE Sept. 1U, 11968 J, FERGASON ET Al. 3,401,262

RADIATION SENSITIVE DISPLAY SYSTEM UTILIZING A CHOLESTERIC LIQUID CRYSTALLINE PHASE MATERIAL Filed June 29, 1965 I 2 Sheets-Sheet 2 l l I 6000 6500 7000 YELLOW RED GREEN ANGSTROMS BLUE ULTRA VIOLET WITNESSES INVENTORS James LI Ferguson 81 Arthur E. Anderson wrung ATTORN RADIATION SENSITIVE DXSPLAY SYSTEM UTILIZ- ING A CHOLESTERIC LIQUID CRYSTALLINE PHASE MATERIAL James L. Fergason, Penn Hills Township, Verona, and

Arthur E. Anderson, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed June 29, 1965, Ser. No. 467,864 8 Claims. ((1 259-83) ABSTRACT UP THE DISCLOSURE The present invention relates to a radiation sensitive display device which includes a heat sensitive display screen of a liquid crystalline material of the cholesteric phase wit-h a radiation sensitive screen incorporating heating elements in thermal contact with the heat sensitive display screen which the heating is due to current flow. The radiation sensitive screen responds to input radiations to cause a current flow which in turn provides heat for impressing a thermal image on the heat sensitive display screen.

This invention relates to apparatus for and methods of detention and display of an electromagnetic wave energy.

There are several systems capable of displaying an electromagnetic Wave pattern or image such as a standard television receiving system. A standard television system relies primarily on adding energy to an electron beam which has been modulated in intensity in accordance with the picture information to be displayed. In large area display devices, practical limitations of beam current density and voltage impose an upper limit on the brightness of the display. Another technique which is currently being studied is electroluminescent phosphor display devices. The electroluminescent display screens have been found to be very ineflicient as far as light ouput from the electroluminescent material and also the associated control means for displaying the image has been found to be extremely complicated and expensive.

The present invention comprises a simplified system for providing a high intensity display. Unlike the conventionally fabricated screens, the display screen utilizes a suitable material such as a liquid crystalline material of the cholesteric phase in which the brightness of the display is proportional to the amount of viewing light including ambient lighting directed onto the screen. The liquid crystalline materials of the cholesteric phase exhi'bits curious changes in light reflecting properties when heated or cooled through a transition region near their melting point. The changes in reflectivity are manifested as changes in color when the viewing source is of white light. The material is substantially colorless at temperatures well above its melting point but as it is cooled and becomes more viscous it goes through a transition region in which it appears blue, then green, then yellow, then red and finally colorless again as viewed in reflective white light. If cooled sufficiently, the viscous liquid is converted to a colorless crystalline solid. The color changes occur at definite, reproducible temperature diflerences within a range of temperatures which may be made relatively broad or narrow by adjusting the formulation of the cholesteric liquid crystalline material.

The optical scattering properties of the cholesteric liquid crystalline layer can be modified by providing in intimate contact therewith elemental driving elements to provide a localized temperature control. Thus for a given cholesteric liquid crystalline material a suitable tem- 3,401,262 Patented Sept. 10, 1968 perature can be found to give an overall background color such as black or red in reflected white light and then by elevating the temperature of localized spots or areas these may be made to appear green or yellow on the contrasting black or red background. By utilizing monochromatic viewing light, single color contrast may be obtained on a given background and brightness alone will vary with temperature change.

The cholesteric film scatters only one wavelength at one angle of the viewing light. Cholesten'c liquid crystal lms absorbs almost no energy so that the light energy directed thereon is either transmitted or reflected. At selected wavelengths and with the viewing light directed onto the film at the normal angle of incident, about fifty percent of the incident radiation is scattered. At larger angles of incidence, the amount of energy reflected or scattered increases and the peak (wavelength of maxi mum scattering) shifts toward the shorter Wavelengths.

If the temperature of the liquid crystalline film is varied, the peak of reflection is shifted to shorter wavelengths with increase in temperature. This temperature eflect is completely reversible. At any temperature the wavelength of maximum scattering depends on the sum of the incident angle of the incident viewing light and the angle of observation.

Liquid crystals of the cholesteric phase have many important optical properties. The cholesteric phase is a state exhibited by many organic materials. It is a member of a large system or class of states called liquid crystalline or mesomorphic indicating a condition or order intermediate between a true crystal and a liquid. For the most part, cholesteric liquid crystals are formed by derivatives of sterols, although they are by no means limited to these compounds. The properties of cholesteric crystals and suitable materials for this invention are more fully discussed in U.S. Patent 3,114,836 by Fergason et al. and assigned to the same assignee as this invention.

It is accordingly, an object of this invention to provide an improved system capable of providing a high intensity display.

It is another object to provide an improved electromagnetic wave detection device.

It is still another object to provide an improved display system capable of providing display in a plurality of colors.

It is another object to provide an improved device for converting electromagnetic wave energy of first wavelength into electromagnetic wave energy of second wavelength.

Briefly, the present invention accomplishes the abovecited objects by providing a heat sensitive display screen of a liquid crystalline material of the cholesteric phase and providing a radiation sensitive screen including heating elements in thermal contact with said heat sensitive display screen in which the heating is due to current flow therethrough. The signal electromagnetic wave is directed onto the radiation sensitive screen which modifies the conductivity and the current therethrough. The change in current flow will produce a change in temperature which is impressed on the heat sensitive screen. The reflective properties of the heat sensitive display screen will be modified and observed by directing a viewing light onto the heat sensitive display screen.

Further objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of the specification.

For better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIG. 1 is a view of a system according to the teachings of this invention;

FIG. 2 is a perspective view of a portion of the screen assembly shown in FIG. 1;

' FIG. 3 is a perspective view similar to FIG. 2 illus trating one modification of the invention;

FIG. 4 is aperspective view similar to FIG. 2 illustrating another modification of the invention; and

FIG. 5 is a graphical representation illustrating the characteristics of the cholesteric liquid crystalline film in the screen structure shown in FIGS. 1 and 2.

Referring in detail to FIG. 1, a display system is illus trated. The 'display system consists of a cathode ray tube for directing a radiation image onto a display screen 12. A suitable lens 11 may be provided for focussing the radiation image onto the screen 12. The other surface of the display screen 12 is illuminated by a viewing light source 14' and a viewer 16 may observe the visible pattern or image on the display screen 12. The cathode ray tube 10 is of any suitable design and includes an electron gun 20 and a luminescent screen 22 of suitable phosphor ma terial which emits visible light in response to electron bombardment by the gun 20. A video signal is fed to the cathode ray tube 10 from a source 24 and may be connected to any suitable electrodes of the electron gun 20. Suitable deflection means, not shown, are provided for scanning of the electron beam generated by the gun 20 over the screen 22. The intensity of the electron beam is modulated by the video signal from the source 24. It may be desirable to utilize a long time persistent phosphor in the screen 22. A radiation image thus is generated on the face of the cathode ray tube 10 representative of the video information applied from the source 24 and this radiation image is directed onto the display screen 12.

The display screen assembly 12 is shown in detail in FIG. 2 and is comprised of the following layers. The display screen assembly 12 is supported by a substrate layer which is also the heat sink for the display screen assembly. Temperature control of the display screen 12 may be obtained by any suitable means such as directing air along one surface of the layer 30 by the air source 28. The support layer 30 is transparent to the input radiation from the cathode ray tube 10 and may be of a suitable material such as glass for visible radiation, barium chloride for infrared radiation or quartz for ultraviolet radiation. In the specific embodiment illustrating a visible radiation input, the thickness of the glass support layer 30 may be of a thickness of about 1 centimeter. The device may be used to convert X-ray pattern to visible light pattern. A thermal barrier layer 32 of a suitable thermal insulating material such as polyethylene terephthalate is provided on one surface of the glass substrate 30. The layer 32 may be secured to the substrate by any suitable adhesive and may be of a thickness of about 25 microns. This material is suitable for both the visible and ultraviolet. In the case of infrared, a layer of polyethylene or polypropylene may be utilized. The thermal barrier layer 32 is to reduce thermal conduction from the sensitive screen portion to the heat sink layer 30. The thermal time constant of the layer 32 should provide a time constant for the sensitive screen layer adjacent thereto of about & of a second.

A layer 34 of photoconductive material is evaporated onto the layer 32 and may be of a thickness of l to 25 microns and of a suitable high impedance photoconductor having a resistance of about 10 ohms-centimeters. Suitable materials for visible light are cadmium sulfide, or arsenic triselenide. A suitable material for ultraviolet is selenium and a suitable material for infrared is lead sulfide.

A layer 36 consisting of a plurality of electrically conductive strips 38 and 40 is provided on the layer 34. The conductive strips 38 are connected to a common bus bar 42 and the strips 40 are connected to a common bus bar 44'. Suitable leads 46 and 48 are connected r'espectively'to the conductive bus bars 42 and 44 for providi'ng a voltage between the condu'ctive'elements 38 and 40: This potential is supplied by a potential source 50 connected across the lead-ins 46 and 48.'The potential source 50 may be an AC or DC potential and may be of the order of about 50 volts. In the specific embodiment shown/the electrode strips 38 and 40 may be evaporated onto the photoconductive layer 34 by well known techniques such as evaporating through a mask to provide strips of thickness of about angstroms. The width of strips 38 and 40 maybe about 50 microns and with a spacing between strips of about 200 microns. An optical isolation layer 52 is provided on' t'he conductive layer 36.'Tl1e optical isolation layer 52'prevents radiations from the light source 14 affecting the photoconductive layer 34; The optical isolation layer '52 may be provided by spraying a coating of" a water soluble black aniline dye of'a thickness of about 1 micron.

Positioned on the optical isolation layer 52 'is'a liquid crystalline layer 54 of cholesteric material and responses to heat by changes in the light reflective properties thereof. Several suitable materials are disclosed in the previously mentioned patent and a specific material for this specific application may be 60 percent by weight of cholesteryl nonanoate, 30 percent by weight oleyl cholesteryl carbonate and 10 percent by weight'of'chol-csteryl benzoate. A protective film 56 may be provided upon the surface of the layer 54 of liquid crystalline material of a suitable material such as polyethylene terephthalate and of a thickness of about 6 microns. it should be noted that the radiation assembly 12 may function as a simple detector simply by providing the photoconductive layer 34, the electrode layer 36 and the liquid crystalline layer 54 In the operation of the device, the cholesteric liquid crystal film 54 and the photoconductive layer 34 are held at a constant operating temperature by the heat sink 3t) and the associated temperature control system 28, in the absence of a projected light image from the cathode ray 10 onto the display assembly 11. The temperature of the liquid crystal layer 54 may be about 32 C. so as .to provide a background color of black to the viewer 16. With the white light source 14 directed onto the liquid crystal layer 54 at the temperature of 32 C. the viewer will simply see a uniform blackbackground due to light reflected from coating 52. When a light image is focused onto the photoconductive layer 34 by means of the cathode ray tube 10 and with the potential source 50 connected across the conductive electrodes 38 and 40, current carries are introduced into the illuminated areas. The amount of illumination directed onto the photoconductive layer 34 determines the amount of current flowing-in the illuminated areas of photoconductive material pr'isit'ioned between the electrode strips 38 and 40; The video source 24 can control the intensity of the electron beam striking the phosphor 22 and accordingly the amount of current and heating by the photoconductive layer- 34. Power dissipation from the resulting photocu-rrent is localized :to only the illuminated areas by the electrode structui'e design, which limits the length of the current path to a fraction of "the smallest resolvable image distance and the high resistance of the photoconductive "layer 34 that is not illuminated. The thermal insulator" 32 between the heat sink 30 and the photoconductive layer 34 permits a temperature rise in the illuminated area of a photocondu'ctive layer 34 and the closely coupled liquid crystalfilm 54. The amount of illumination determines the amount of current flow and heat generated. Due to this temperature rise, the reflection band of the liquid crystal film 54 in those areas is shifted toward a shorter wavelength part of the spectrum. This is illustrated in FIG. 3 and forexample if the temperature is raised to 335 C. the reflecte color will be red At 34.1 C. the color will be yellowfat 35.5 C. the color will be green, at 37 C. the color will be blue and at 40 C. the color will be in the ultraviolet region and appear black to the observer.

The optical isolation layer 52 between the photoconductive layer 34 and the liquid crystal film 54 permits heat transfer between the two. The layer 52 blocks light from the light source 14 used for illuminating of the viewing side of the panel from effecting the photoconductive layer 34. This isolation layer 52 also serves as a black background for the liquid crystal film 54.

As indicated if the display is illuminated with white light from the source 14, the liquid crystal film 54 provides a color display of the temperature pattern or image. Such a display is inherently compatible with high ambient illumination because reflective light, which is proportional to the incident light rather than internal generative light, produces the image. In this manner a large amplification of the input signal from the cathode ray may be obtained so as to obtain an amplification greater than 100. If the display is illuminated with a monochromatic light from the source 14 rather than white light, a single color display with gray scale capabilities results.

The temperature rise required to activate the liquid crystal is created when light impinges on the photoconductive layer 34, resulting in photocurrent. The photocurrent is generated by applying a voltage from source 50 across the interlaced comb-like electrode system consisting of alternate conductors 38 and 40. The conductive electrode system 38 and 40 consists of parallel conductors Whose width is a fraction of a spacing to obtain maximum active surface area and whose spacing is determined by the desired display panel resolution. The length of the photoconductive current path, which is equal to the electrode spacing, must be a fraction of the smallest resolvable distance on the display.

The liquid crystal film 54 may be of various materials and is selected for the required sensitivity and the operating temperature range. The operating temperature may be chosen arbitrarily Within the limits compatible with the temperature control method. The sensitivity of the film is determined by the application of the display. A liquid crystal film with high temperature sensitivity will result in a display with high overall sensitivity, but will also require a much better temperature control system and layer uniformity to eliminate color variations in the reflected light image due to small temperature variations over the surface :and to sensitivity and normal resistance variations in the various layers.

The protective film 56 is provided on the liquid crystal to reduce contamination of the liquid crystal film by dust and chemicals in the atmosphere. It also minimizes temperature variations due to air flow in front of the image intensifier panel 12. If chosen properly, the film material provides additional enhancement for the molecular alignment in the liquid crystal film and does not contribute significantly to lateral heat spread. The chemical composition of the film 56 and the optical isolation layer 52 must prevent contaminating substances from being released into the liquid crystals, whose optical properties are very sensitive to the presence of various chemical substances as well as temperature variations.

In FIG. 3 a modified photoconductive and electrode system is illustrated. In this embodiment, two conductive bus bars 60 and 62 are provided and parallel strips 64 of photoconductive material are evaporated across the space so as to be in electrical contact with both of the conductive bus bars 60 and 62. The photoconductive strips 64 may be provided with conductive elements 66 interspersed in the photoconductive strips to limit the range of carriers to provide high resolution. In this type of configuration a lower impedance photoconductive material may be utilized, that is one having a dark resistance, of less than 10 ohms-centimeters and such materials as lead sulfide, lead telluride and indium antimony. Here again, the conductive bus bars 60 and 62 may be evaporated onto the thermal insulating layer 32 and the photoconductive elements 64 evaporated onto the substrate 32 by well known procedures in which evaporation is provided through a mask. The source 50 would be connected to the bus bars 60 and 62. The remainder of the device is similar to that already described and the operation is also similar and will not be described again.

FIG. 4 illustrates another possible modification of the photoconductive and electrode system in which a large area of electrically conductive layer 70 is deposited on the thermal barrier 32. A layer 72 of photoconductive material 70 is deposited on the layer 70 and another electrically conductive layer 74 is provided on the photoconductive layer 72. The optical isolation layer 52 would be provided on the conductive layer 74. Here again, the operation is similar to that previously described. The layer 70 should be transmissive to the input radiations and may be of a material such as stannic oxide. The source 50 would be connected to the electrodes 70 and 74. If desired, the photosensitive element may be used to control the current through a resistive heating element in contact with the heat sensitive screen. It is also possible to convert sound energy into visible energy by detecting the sound on an element Whose resistance is modified and therefore permitting control of current and associated through the detecting element per se or an associated resistive heating element in thermal contact with the heat sensitive screen.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements slhown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

.1. A radiation display system comprising a light control element, said light control element including a heat sensitive screen, said heat sensitive screen exhibiting the property of different optical properties at different temperatures, means for directing viewing light onto a first surface of said light control element to illuminate said heat sensitive screen, an input radiation sensitive layer positioned on a second surface of said heat sensitive screen, a radiation input source, means for directing said radiation input onto said radiation sensitive layer to modify the current conductive properties thereof, electrically conductive means associated with said radiation sensitive layer to induce a current in radiation exposed areas of said radiation sensitive layer to heat areas of said [heat sensitive screen associated with said radiation exposed areas of said radiation sensitive layer to vary the optical properties of said heat sensitive screen and provide a light image corresponding to said radiation input.

2. A radiation display system comprising a light control element, said light control element including a heat sensitive screen, said heat sensitive screen exhibiting the property of different wavelength reflection of radiations at different temperatures, means for directing viewing radiation onto one side of said light control element to illuminate said heat sensitive screen, electrical current heating means in thermal contact with said heat sensitive layer, and radiation sensitive means electrically connected to said heating means, said radiation sensitive means responsive to a signal radiation to modify the current flowing through said heat-ing means and thereby heat said heat sensitive screen and modify the radiation reflection properties of said screen.

3. A display system comprising a radiation control element, said radiation control element including a liquid crystalline material and exhibiting the property of different optical properties at different values of excitation, means for directing viewing light onto a first surface of said radiation control element to illuminate said radiation control element, a radiation sensitive material positioned on a second surface'of said heatsensitive screen, and means for "directing input 7 radiations onto said radiation "sensitive material to cause an increase in the electrical conductivity properties thereof. g

4. A display system comprising a radiation Control element, said radiation control element comprised of a heat sensitive layer including a cholesteric liquid crystalline material, said heat sensitive layer exhibting the property of different reflection properties of radiation at different temperatures, means for directing viewing radiations'onto said heat'sensitive layer, a photoconductive layer positioned on one side of said heat sensitive layer and in thermal contact with'said heat sensitive layer, means for directing input signal radiation onto said photoconductive layer to modifythe conductivity properties thereof, electrically conductive means associated with said photoconductive layer to induce current flow through said photoconductive layer in accordance with the conductivity thereof and cause heating of said heat sensitive layer to vary the reflection properties of said heat sensitive layer to said viewing radiation and provide an amplified display of'said input signal radiation.

5. radiation converter comprising a layer of cholesteric liquid crystalline material, said liquid crystalline layer exhibiting the property of different reflection properties of light at different temperatures, a radiation source for directing viewing illumination onto said liquid crystalline layer, a photoconductive layer positioned on the side of said liquid crystalline layer remote to said radiation source, and in thermal contact with said liquid crystalline layer, means for directing an input radiation pattern onto said photoconductive layer to induce corresponding conductivity pattern therein, means associated with said photoconductive layer to induce current flow through said photoconductive layer in accordance with said conductivity pattern and impress a heat pattern on said liquid crystalline layer to vary the reflection properties of said liquid crystalline layer to said viewing illumination and provide an output radiation pattern corresponding to said input radiation.

6. A display system comprising a light control device including a layer of liquid crystalline material, said liquid crystalline material exhibiting the property of different light reflecting properties at different temperatures, a layer of photoconductive material in thermal contact with said press a conductivity image therein corresponding to said radiation image and establish a heat image within said photoconductive layer due to current flow therein substantially corresponding to said radiation image and impressing said heat image on said liquid crystalline layer, means for directing a light source onto said liquid crystalline-layer to obtain a light image corresponding to said iheat image due 1 to changes in light reflection properties of said liquid crystalline layer due to changes in temperature. g

7. A display system comprising a light control device including a layer of cholesteric liquid crystalline material, said cholesteric liquid crystalline material exhibiting the property of different optical properties at'dilferent temperatures and a layer of photoconductive material in thermal contact with said cholesteric liquid crystalline layer.

8. A display system comprising a light control device including a layer of liquid crystalline material, said liquid crystalline material exhibiting the property of different light reflecting properties at different temperatures, a layer of photoconductive material in thermal contact with said liquid crystalline layer, means for impressing a voltage across said photoconductive layer, means for directing a signal modulated radiation beam over said photoconductive layer to impress a conductivity image therein corresponding to said modulated radiation beam and establish a heat image within said photoconductive layer due to current flow therein and impressing said heat image on said liquid crystalline layer, means for directing a light source onto said liquid crystalline layer to obtain a light image corresponding to said heat image due to changes in light reflection properties of said liquid crystalline layer due to changes in temperature.

References Cited UNITED STATES PATENTS 2,788,452 4/1957 Sternglass 250-83.3

3,114,836 12/1963 Fergason et al. 25083.3

ARCHIE R.BORCHELT, Primary Examiner. I 

